Resin-fiber composite materials are utilized in a variety of applications including the aerospace industry, for example. Structures which are constructed of resin-fiber composite materials may be exposed to heat, which may affect the composite materials in various ways. These heat-induced effects may include chemical effect in which changes such as oxidation, material loss and the breaking and/or forming of chemical bonds occurs in the polymer chemical structure of the composite materials. Resin decomposition, strength effect, charring and fiber decomposition of the composite materials may occur at increasing temperatures.
Composite materials and epoxy-based surfacing films are typically cured under a vacuum at 250-350° F. for several hours. Repairs to the composite materials or surfacing films can become necessary if voids are found during an NDI inspection or if a flaw is found. These repairs may require the use of heating blankets that are notorious for having hot spots. Thus, during the repair some areas of the composite material or surfacing film may become overheated; in the case of epoxy-based surfacing films, a color change (dark or blackened) may be noticeable. Thermal effect for in-service airplanes can occur from engine and run-way fires, electrical problems, lightening strikes, and other causes. Thermal effect may affect the mechanical and structural integrity of composite materials and surfacing films.
Repair or removal of heat-effected composite materials or surfacing films on a structure may involve first determining the degree of harmful heat effect to the composite materials or surfacing films. Although determining the degree of heat effect to composite materials or surfacing films may be performed by visual inspection, heat effect may not be visually apparent. Previous methods of determining the presence and extent of heat effect in composite materials and surfacing films includes obtaining an infrared spectrum of a heat-affected composite standard and correlating the infrared spectrum obtained from the composite standard with the degree of thermal effect of the composite standard. An infrared spectrum obtained from the composite material or surfacing film the heat effect of which is in question can then be compared to the infrared spectrum obtained from the composite standard to determine the presence and assess the degree of thermal effect in the composite material or surfacing film. Current methods of determining the presence and extent of heat effect in composite materials and surfacing films include obtaining the spectra of a series of heat-affected composite standards and building a multivariate model with the spectra of those standards. An infrared spectrum obtained from the composite material or surfacing film which is in question can then be predicted by the multivariate model to determine the presence and assess the degree of thermal effect in the composite material or surfacing film.
U.S. Pat. No. 7,115,869 discloses a method for measurement of composite heat effect with infrared spectroscopy by which an amount of heat exposure to a resin-fiber composite substrate is determined. According to the method, a value of infrared energy reflected by a composite substrate is determined. The value of infrared energy which is reflected or absorbed is correlated to a degree or amount of heat exposure of the composite substrate. The thermal effect measurement method of the present disclosure is more sensitive than that which is disclosed in U.S. Pat. No. 7,115,869 and is richer in spectral detail, providing a better indication of whether thermal effect has occurred in a composite material as well as the extent of effect. Moreover, the method can be implemented using a hand-held spectrometer which facilitates convenience and flexibility in measuring the possibility or degree of thermal effect in the composite material in question.
Calibration of infrared spectrometers to residual strength in composite materials or surfacing films correlates the resin condition as read from the infrared spectrum to the residual strength of the material or film which degrades as the resin degrades with progressively increasing temperatures. Therefore, the infrared sensors may be calibrated using time-controlled thermal soak standards which are obtained by exposing various composite materials or surfacing film controls to various temperatures for a particular time period such as one hour, for example. One method of preparing the standards includes placing the standards in an oven which is calibrated periodically and monitored continuously using multiple thermocouples as the composite materials are thermally exposed. Furthermore, thermal effect to composite materials or surfacing films may not follow a linear course. The most useful parameters for thermal effect are those that the repair process person can understand and use as criteria for removal of effect material. Under circumstances in which it is desirable to remove all weakened material, a measure of the residual strength in the material is best. Multivariate calibration allows calibration to such physical properties under circumstances in which previous calibration methods were insufficient.
Therefore, a method for accurately assessing thermal effect in a composite material or surfacing film using mid-infrared spectroscopy is needed.